Polyamines are essential growth factors. As such, polyamine homeostasis is tightly regulated by synthetic and catabolic enzymes as well as transport systems. Certain cancers, like colorectal cancer, have a high need for intracellular polyamines and can be targeted via polyamine depletion strategies. Indeed, there are four major targets (polyamine synthesis, catabolism, import and export) which can initiate this outcome. Of these four, the synthetic and catabolic enzymes are well understood. However, the transport systems associated with polyamine import is a ‘black box’.
Knowledge of the polyamine transport system is a critical gap in our knowledge base. Previous efforts to deplete cells of polyamines via specific inhibitors of polyamine synthesis do not block these cells from recovering via import of polyamines from the extracellular milieu. For example, treatment of HCT-116 colon cells with difluoromethylornithine (DFMO) inhibits ornithine decarboxylase (ODC) and limits putrescine production, i.e., one of the three native polyamines (Roy, U. K.; Rial, N. S.; Kachel, K. L.; Gerner, E. W. Activated K-RAS increases polyamine uptake in human colon cancer cells through modulation of caveolar endocytosis. Mol Carcinog. 2008, 47, 538-553). However, this is merely a cytostatic effect as cells can continue to survive via their existing polyamine pools and import. Indeed, cell growth continues, when DFMO is removed.
While the genes associated with polyamine transport are known in E. coli, yeast and Leishmania, they are unknown in mammals. There are titillating reports by Belting and Gerner, which suggest that caveolin-dependent lipid rafts may be involved in mammals. The Gerner group has had some success treating colon cancers with DFMO (which blocks ODC and polyamine synthesis) in combination with non-steroidal anti-inflammatory drugs (NSAIDs), which facilitate polyamine export by an unknown process (Roy, U. K.; Rial, N. S.; Kachel, K. L.; Gerner, E. W. Activated K-RAS increases polyamine uptake in human colon cancer cells through modulation of caveolar endocytosis. Mol Carcinog. 2008, 47, 538-553). The Belting group demonstrated polyamine uptake involves binding to proteoglycans. (refs: (a) Belting, M.; Mani, K.; Jönsson, M.; Cheng, F.; Sandgren, S.; Jonsson, S.; Ding, K.; Delcros, J.-G.; Fransson, L.-A. Glypican-1 Is a Vehicle for Polyamine Uptake in Mammalian cells: A Pivotal role for Nitrosothiol-derived nitric oxide. J. Biol. Chem. 2003, 278, No. 47, 47181-47189; b) Belting, M.; Persson, S.; Fransson, L.-A. Proteoglycan involvement in polyamine uptake. Biochem. J. 1999, 338, 317-323; c) Belting, M.; Borsig, L.; Fuster, M. M.; Brown, J. R.; Persson, L.; Fransson, L.-A.; Esko, J. D. Tumor attenuation by combined heparan sulfate and polyamine depletion. PNAS 2002, 99, No. 1, 371-376.)
The Porter group has also investigated the induction of spermidine/spermine acetyl transferase (SSAT), which tags polyamines for export via N-acetylation, as another approach to deplete cells of native polyamines (Porter C W, Ganis B, Libby P R, Bergeron R J. Correlations between polyamine analogue-induced increases in spermidine/spermine N1-acetyltransferase activity, polyamine pool depletion, and growth inhibition in human melanoma cell lines. Cancer Res. 1991 Jul. 15; 51(14):3715-3720). SSAT action increases the amount of exportable polyamines (N1-acetylpolyamines) available. Without knowledge of the polyamine transporter (PAT) and a technology to inhibit its uptake of extracellular polyamines, the above anti-cancer strategies will always have to contend with polyamine uptake. Uptake provides a rescue mechanism from intracellular polyamine depletion. There is an urgent need therefore to develop PAT inhibitors which block this ‘rescue pathway’ of polyamine import.